Energy level decay processes in Ho3+-doped tellurite glass relevant to the 3 μm transition

The primary excited state decay processes relating to the I-5(6) -> I-5(7) similar to 2.9 mu m laser transition in singly Ho3+-doped tellurite (TZBG) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the I-5(6) energy level at 1151 nm and I-5(7) energy level at 1958 nm has established that the rate of energy transfer up-conversion between holmium ions excited to the I-5(7) level is negligible for Ho3+ concentrations up to 4 mol. %. Excited state absorption was not observed from either the I-5(7) or I-5(6) levels and the luminescence from the I-5(7) and I-5(6) energy levels was measured to peak at similar to 2050 nm and, similar to 2930 nm, respectively. The I-5(6) level has a low luminescence efficiency of similar to 8.9% due to strong nonradiative multiphonon relaxation. In contrast, decay from the I-5(7) level is essentially fully radiative. A linear decrease in the decay time of the I-5(6) level with Ho3+ concentration augmentation results from energy transfer to OH- ions in the glass (with N-OH similar to 8.2 x 10(17) ions cm(-3)) and reduces the luminescence efficiency of the I-5(6) level to 8% for [Ho3+] = 4 mol. %. Numerical simulation of a fiber laser incorporating 4 mol. % Ho3+ showed that a population inversion of similar to 7.8% is reached for square pulses of 100 mu s duration and a repetition frequency of 20 Hz at a moderate pump intensity of 418 kW cm(-2) if energy transfer to OH- radicals is neglected. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3587476]